In the prior art, in paper or board machines, a stock suspension jet is discharged out of a slice opening onto a forming wire or into a forming gap between forming wires. The cross-direction profile of the slice opening determines the profile of the stock jet. The profile of the slice opening is regulated such that it is also possible to compensate for faults in the stock jet that arise in or before the headbox.
A headbox, in particular a multi-layer headbox, is a difficult item to regulate because the stock jet formation process involves a number of different parameters, which have various cross effects, which effects may further depend on the raw-materials that are used, on the grade of paper that is being produced, and/or on the geometry of the headbox and on the mode of running of the paper machine. However, in the manufacture of a paper product of good quality, in particular of printing and writing paper, in particular when the multi-layer technique is applied, the ratios of speed and flow quantities between the different stock flows and the settings of these parameters in relation to the wire speed must be sufficiently precisely controllable.
From the prior art, a system for the control of the basis weight profile of the headbox in a paper machine is known. This system comprises an angular-gear/stepping motor actuator, by whose means the top slice bar which regulates the slice profile is controlled by adjusting spindles attached to the slice bar at a spacing of about 10 cm to about 15 cm. The spindles are displaced by the actuator placed at one of their ends. The profiling of the top slice bar of the headbox usually takes place so that each regulation gear is controlled separately in a sequence of treatment taking place one after the other. In order that the positioning could be carried out with the required precision of about 10 .mu.m, an electronic system for measurement of the locations of the regulation spindles is also needed.
Errors in the fiber orientation in a paper web arise mainly from the following causes. A smaller amount of stock flows at the edges of the stock flow channel in the headbox. This edge effect causes a very strong linear distortion in the profile. Errors of profile in the turbulence generator in the headbox usually cause a non-linear distortion inside the lateral areas of the flow channels. The acceleration produced in the slice cone of the headbox equalizes the profile errors in the main flow, but it is exactly that effect that produces the cross-direction flow. Errors of orientation in the paper web are indirectly also caused by the operation of the dryer section because, during drying, the paper web can shrink unevenly in the cross direction so that the lateral areas shrink to a considerably greater extent than the middle area. Attempts are made to compensate for the unevenness of the basis weight profile caused by the drying shrinkage by means of crown formation of the slice opening so that the slice opening is thicker in the middle area of the stock jet. This, however, results in cross-direction flows in the slice jet and further in the wire part, which again causes distortion of the fiber orientation. The same phenomenon also affects the cross-direction strength profiles of the web.
In the prior art, a method is known for on-line regulation and measurement of the fiber orientation in a web that is produced by means of a paper machine from the current assignee's Finnish Patent No. 81,848 (corresponding to European Patent No. 408,894). In the method of regulation of a paper machine described in this patent, with the machine configuration and the parameters at each particular time applied in the paper machine to be regulated, data is collected concerning the relationship between the cross-direction distribution of fiber orientation and the basis weight distribution of the web that is being produced so that response runs are carried out with the paper machine in different states of operation of the machine. The data on the relationship that were obtained in the stage defined above is stored in the memory of the computer or equivalent included in the system of regulation of the paper machine. While making use of this data on the relationship, by means of the system of regulation, the distribution of the fiber orientation in the web that is being produced is corrected by regulating the cross-direction profile of the slice opening or equivalent of the headbox. The method of measurement of this patent comprises the two first-mentioned stages and further, a stage in which, while making use of the data on the relationship, the distribution of the fiber orientation in the web is expressed based on the measurement of the cross-direction basis weight profile of the paper web. Response runs are carried out in order to determine the relationship between fiber orientation and basis weight at each particular time, in connection with which response runs, the distribution of the fiber orientation is measured in the laboratory by taking samples out of the web that is being produced. From the samples, the fiber orientations are determined with sufficiently dense spacing in the cross direction of the web by making use of prior art commercial methods and apparatus of laboratory measurement or tests of diagonal tensile strength. Such a high number of response runs and related series of laboratory measurements are carried out that, while average values are computed from the measurement results, a sufficiently good explanatory quality is obtained between the basis weight profile and the fiber orientation profile.
In the manufacture of paper, one of the most important factors that affect the functional properties of paper is the so-called machine-direction/cross-direction ratio of strength. This ratio is controlled by regulating the difference in speed between the jet speed at the headbox and the speed of the forming wire or wires or the ratio of these speeds (hereinafter referred to as the j/w. ratio). Generally, the wire speed is kept invariable, and the regulation takes place by means of regulation of the headbox jet speed.
The ratio of the tensile strengths of paper in the machine direction and in the cross direction is regulated in a way known from the prior art. For example, the tensile strengths are measured in the laboratory at regular intervals, typically from a sample of paper taken from each machine reel. If the tensile strengths are not at the specified level, the j/w ratio at the headbox is usually altered. The tensile strengths can also be affected by means of properties of the stock, but the j/w ratio is the primary parameter of regulation. It is considerably more difficult to act upon the properties of the stock, because such changes, for example grinding and ratios of different stock components, affect almost all quality factors of paper.
As known in the prior art, the headbox jet speed is monitored by measuring the static pressure at the beginning of the slice cone and by converting the pressure value, by means of computations, into the jet speed. In the computations, it is necessary to take into account the geometry of the headbox and the friction and turbulence losses caused by the walls and by possible vanes. In particular, it is very difficult to produce a mathematical model for the losses arising from vanes, which results in problems in the control of the jet speed when changes take place in the running parameters of the headbox, such as-the geometry of the slice part, or when the grade that is produced or the raw-material is changed, in which case a considerable proportion of the paper can be lost in connection with a change of grade before the quality can be regulated and stabilized at an appropriate level. In the prior art, devices for direct measurement of the speed of the discharge jet are also known, but fitting of such devices in the area of the forming gap, in which the space is very limited, is quite difficult and, moreover, the detectors disturb the stability of the jet.
Some of the critical parameters of a paper web are the fiber orientation ratio (hereinafter referred to as the fiber ratio) and the orientation angle, because the fiber ratio affects the properties of strength of paper and board so that the tensile strength of the web in the plane of the paper is substantially higher in the direction of the fibers as compared with the direction perpendicular to the fiber direction. The MD/CD ratio of tensile strength (machine direction/cross direction) of the web is determined by the orientation ratio, i.e., the fiber ratio O.sub.max /-O.sub.min, and the definition of this ratio will be dealt with in more detail in connection with the description related to FIG. 3. The fiber ratio also affects the runnability of a paper machine and a printing and copying machine. For this reason, it is a recognized objective is that the paper should be stronger in the machine direction than in the cross direction, in which case, for example in a printing machine, a paper web that runs in its machine direction endures tension to a greater extent than in the cross direction. Typically, the MD/CD ratio of tensile strength is in a range of from about 0.9 to about 4.5, in the case of printing and writing papers, and more particularly, in a range of from about 1.5 to about 3.5.
In a paper machine, in the web formation, the fiber ratio is affected above all by the j/w ratio. If the wire speed differs from the jet speed, the fibers have a tendency to orient in the machine direction to a greater extent than in the cross direction, in which case, the tensile strength of the web in the machine direction is increased, which is usually desirable. If the slice jet has a cross-direction speed component when it is discharged from the slice opening of the headbox, the principal direction of orientation can differ from the machine direction, in which case, the orientation angle (angle .alpha., FIG. 3) becomes larger than zero.
Recently, detectors and systems of measurement of tensile strength/tensile rigidity and of fiber orientation have been introduced in the market. With respect to these, reference is made, by way of example, to the paper in the journal Svensk Papperstidning/Nordisk Cellulosa No. 6:1997, pages 64-66, Gunnar Lindblad: "Infraljudsmatning - ett nytt satt att styra pappersmaskinen" (`Infrasonic measurement--a new mode of controlling the paper machine`).
Further, with respect to the prior art related to the present invention, reference is made to the following published patent applications and patents: International Publication No. WO 97/01088 and U.S. Pat. Nos. 4,133,713, 4,151,415 and 5,145,560.